Aluminum electroforming



Patented Oct. 26, 1954 UNITED STATES PATENT OFFICE ALUMINUMELECTROFORMIN G mesne assignments,

tion of Delaware to ment Corporation, Colu The Battelle Developrnbus,Ohio, a corpora- No Drawing. Application November 2, 1951, Serial No.254,656

8 Claims. 1

This invention relates to coating with aluminum. More particularly, itrelates to an improved process and bath for depositing aluminum onmetal, or other electrically conductive surfaces, and to a bright,smooth aluminum coating produced thereby.

The physical properties of aluminum, particularly its resistance tocorrosion, tend to make it highly desirable for use as a structuralmaterial. However, the low tensile strength prevents widespread usage inpure form. Accordingly, many attempts have been made to form a coatingof aluminum on steel or other suitable construction materials, whereinthe aluminum coating will serve to protect the surface of the basematerial. Because of the physical properties of aluminum, it is alsouseful as a decorative, corrosion-resistant coating.

This improved process for electrodepositing aluminum is also useful inelectroforming articles of complex shape. A low-melting alloy can becast, or otherwise formed, into the desired shape, and then coated withaluminum. The base material is melted out, leaving a light-weight shell.This procedure is particularly useful for manufacturing radar waveguides and comparable electronic equipment. It has not been possible tomake such articles by prior-art methods of electrodepositing aluminum.

Although several processes for electrodepositing aluminum have beendescribed in the prior art, it has not been possible to produce densecoatings thicker than about 0.002 inch, since any excess of depositedmetal consisted entirely of nonadherent powder, sponge or trees. As aresult, the surfaces were not smooth and bright as the term is commonlyused in the art.

Another difilculty encountered in prior-art methods of electrodepositingaluminum is the extremely short life of the bath. After only a few daysuse, the bath deteriorates causing nonadherent deposits that arecompletely spongelike.

In Patent No. 2,446,331, Hurley has disclosed an electrolyte for use inthe electrodeposition of aluminum. This bath comprises an anhydrousaluminum halide and a N-alkyl pyridinium halide. Wier and Hurley, inPatent No. 2,446,349, show the addition to this electrolyte of a liquidaromatic hydrocarbon, such as benzene, toluene, xylene, and the like.Also, Wier, in Patent No. 2,446,350, discloses the further improvementof superimposing an alternating current on the direct current used inthe plating bath.

In the process of Patents Nos. 2,446,349 and l .or benzene.

2,448,350, a finite amount of toluene was miscible with. the aluminumhalide-alkyl-pyridinium halide fusion, and any excess amount ofhydrocarbon floated in a layer on top of the fusion.

Copending application, Serial No. 254,655, filed November 2, 1951,discloses that if an excess amount of a liquid aromatic hydrocarbon isadded, and if the bath is continuously agitated so that the excesshydrocarbon is dispersed throughout the electrolyte, a thicker platehaving better properties can be deposited.

It has now been found that the addition of certain organic additionagents will result in the deposition of thick, dense and ductilealuminum coating which also has a greater hardness than prior-artcoatings. In addition, this coating is bright and has a high degree ofcrystal orientation.

It is accordingly one of the objects of this invention to provide abright aluminum coating which is dense and ductile.

Another object is to provide an aluminum coating having a high degree ofcrystal orientation.

It is a further object to provide an improved method of depositingaluminum on a metal, or other electrically conductive surface.

Yet another object is to provide an improved, long-life electrolyte fordepositing aluminum on a metal, or other electrically conductivesurface.

In general, the bath for depositing the aluminum coating comprises anonaqueous solution of a fusion of an aluminum halide and a quaternarysalt of nitrogen. This fusion is dissolved in an excess of a liquidaromatic hydrocarbon, an organic addition agent is added, and the bathis agitated continuously during the plating operation.

In preparing the fusion mixture for the bath, any aluminum halide may beused. The aluminum halide is fused with a quaternary salt of nitrogen,preferably a pyridinium salt. Very satisfactory results have beenobtained by using a N-alkyl pyridinium halide, such as methyl or ethylpyridinium halide. Proportions of about two mols of the aluminum halideto about one mol of the nitrogen salt will form a low-melting eutectic.

The solvent for the fusion mixture is a liquid aromatic hydrocarbonmixture, such as toluene A sufficient amount of solvent is used tocompletely dissolve the fusion product and also form a layer of excesssolvent on top of the fusion solution. Although any excess of solventwill aid in imparting desired properties to the bath, an excess of lessthan 23 per cent,

per volume of the total bath will result in a plating which is lesssmooth, dense and ductile, than when a larger excess is used. If overabout 45 per cent excess is used, it cannot be completely dispersed. Thebath also tends to become nonconductive when more than about 45 per centsolvent is used. The most satisfactory platings were formed from bathshaving an excess of solvent in the range of from 35 to 40 per cent.

Suitable organic addition agents, which aid in giving desired propertiesto the bath, include methyl tert-butyl ether, ethyl ether, diphenyloxide, dimethyl aniline, di-o-tolylurea, di-o-tolyL thiourea anddichloroethylene.

The introduction of a small amount of the organic addition agent issufiicient to produce improved coatings. Although the exact amount will,of course, vary with the size and composition of the bath, currentdensities, placement of electrodes, amount of agitation, etc., very goodresuits were obtained by an amount equal to about 1.0 to 2.2 per cent byvolume of the bath.

The plating cells may be constructed of glass, ceramics, plastics, orany other suitable material which will not react with the bath. It isalso desirable to provide a cover for the cell whenever possible inorder to prevent the entry of impurities into the bath.

As has been heretofore disclosed, it is important that the bath beagitated during the plat-- ing operation in order to disperse the exceshydrocarbon solvent. Although any suitable means may be used for thisagitation, the most satisfactory results have been obtained by bubblingan inert gas through the bath. Nitrogen, argon, or any other inert gas,may be used. However, care must be taken that no oxygen is present,since this causes the bath to degenerate. It is desirable to remove anyoxygen present in the gas before passing it into the bath.

The flow rate of the gas depends, to a great extent, on the surface areaof the plating bath. However, best results are obtained when the rate isslightly greater than the minimum fiow needed to disperse the excesssolvent. A rate of about 0.2 cu. ft. per hour per sq. in. of surfacearea provides satisfactory dispersion.

Dispersion of the excess hydrocarbon may also be accomplished by meanssupersonic vibration from a quartz crystal.

The base material to be coated forms the cathode in the bath. Materialssuitable for coating include steel, copper, silver, lead-, zinc-, andcadmium-base alloys, and similar materials. The anodes are aluminum.High-purity aluminum, such as 99.9% or 99.99%, forms better coatingsthan, for example, 2-S aluminum. These anodes dissolve with a currentefficiency nearly equal to 100 per cent.

During the coating process, the bath should be maintained within atemperature range of about 80 F. to about 100 F. At temperaturesappreciably lower than 80 F., it has been found that the resultingcoating tends to be brittle and cracked. The optimum temperature forcoating is from about 83 F. to about 87 F.

For best results in coating, an alternating current should be imposed onthe direct current passed through the bath. Successful results have beenobtained with cathode densities of 5 to 25 amp. per sq. it, D. C., and2.5 to 50 amp. per sq. ft., A. 0. Preferred current densities are in therange of 8 to 10 amp. per sq. it. D. C., and 10 to 20 amp. per sq. it.A. C. Less than 6 volts are needed.

The high finish on both surfaces of the aluminum coating also makes itdesirable for use in forming hollow articles of complex shapes, such aswave guides. A low-melting alloy is cast in the shape desired for thefinished hollow article. This mandrel is surface-finished, and preferably coated with a copper plate to facilitate deposition of a smoothaluminum plate. Aluminum is then electrodeposited, utilizing the processand bath heretofore disclosed. The mandrel is melted cut and the copperplate dissolved from the inner surfaces of the hollow article. Theresulting article has a uniform thickness, is light in weight, andcompares favorably in physical properties to a heavy metal electroform.

Alternatively, a cast or machined nonmetallic solid, such as a plastic,can be used as a base by surfacing it with silver by any of thewell-known chemical reduction methods and electrodepositing aluminumthereon. The plastic is then dissolved by means of a suitable solvent.If desired, the silver liner need not be removed from the inner surfaceof the electroforrned aluminum article.

The following examples will serve to illustrate the invention withgreater particularity:

Example I A mixture in the ratio of 2 mols of aluminum chloride and 1mol of ethyl pyridinium bromide was fused at 400 F. 3000 grams of thisfusion product were added to 7000 cc. of toluene. About 3700 cc. oftoluene were required to dissolve the fusion product, leaving an excessof 3,300 cc. of toluene. 96 cc. of methyl tert-butyl ether were added.The total volume of the bath was about 9100 0C.

The bath was placed in a glass plating tank. A current was passedthrough the bath between a copper-base material, as the cathode, and analuminum anode. The base material was a waveguide mandrel having arectangular cros section. A source of direct current was connected fromthe anode to the cathode, and a source of BO-cycle alternating currentwas connected in series with the direct current. The cathode density was10 amp. per sq. ft. for the D. C. current and 20 amp. per sq. ft. forthe A. C. current. Anode densities were 2.0 and 4.0 amp. per sq. ft. forthe D. C. and A. C. currents, respectively. During the plating the bathwas maintained at a temperature of F. The excess toluene was dispersedby bubbling high purity nitrogen through the bath at a rate of 7.5 cu.ft. per hour. After 49 hours, an electroplate about 0.028-inch thick wasformed on the base material. The coating was smooth, ductile and bright,and had a high degree of crystal orientation with the (-110) crystalplanes being parallel to the plane of the Example [I '75 grams of thefusion product in Example I were dissolved in 200 cc. of toluene,resulting in an excess of toluene of about '00., or 38 per cent of thetotal volume which was 250 cc. 2.5 cc. of ethyl ether were added to thebath and it was placed in a glass plating tank, Using a cur rent sourcesimilar to Example I, the current was passed from an aluminum anode to acopper-base material serving as a cathode. The cathode current densitywas 10 amp. per sq. ft. for both A. C. and D. C. current, and the "anodecurrent density was 2 amp. per sq. ft. for both A. C. and D. C. current.The bath temperature was maintained at 85 F., and the excess toluene wasdispersed by a nitrogen flow of 1 cu. ft. per hour. After 36 hours, anelectroplate 0.020-inch thick was formed. This foil was tough, ductileand dense.

Example III A bath similar to that used in Example II was prepared,except that 2.5 cc. of diphenyl oxide were used as the organic additive.The same plating equipment and electrical circuit were used as inExample II. The current densities were 10.0 and 13.0 amp. per sq. ft.,respectively, for the D. C. and A. C. at the cathode, and 5.0 and 6.5amp. per sq. ft, respectively, for the D. C. and A. C. at the anode.After plating for 12.5 hours at a bath temperature of 100 F. and using aflow of 3 cu. ft. per hour of nitrogen to disperse excess toluene, anelectroplate 0.006-inch thick was formed. This deposit was dense,smooth, and bright, and had good ductility.

Example IV A mixture of 551 g. of ethyl pyridinium bromide and 752 g. ofaluminum chloride were fused at 400 F. for 30 minutes. 80 g. of thefusion product were added to 200 cc. of toluene. 2.6 cc. of dimethylaniline were added. The same plating equipment and electrical circuitwere used as in Example II, except that the cathode was a coppercoatedwave-guide mandrel. The current densities were 27 amp. per sq. ft. forboth D. C. and A. C. at the cathode and 5 amp. per sq. ft. for both D.C. and A. C. at the anode. The voltage was about 2.3 volts. Afterplating for 42 hours at a bath temperature of 100 F., and using a flowof 1.3 cu. ft. per hour of nitrogen to disperse the excess toluene, anelectroplate 0.018- inch thick was deposited. When the mandrel wasremoved the wave-guide segment was found to have a smooth, densesurface, free of cracks or other discontinuities. It also possessed goodductility. The hardness of the electrodeposited aluminum was 33 to 54Knoop. The crystals were highly oriented with the (100) planes parallelwith the cathode surface.

Example V The same bath was used as in Example IV except that 2.5 g. ofdi-o-tolylthiourea were substituted as the organic additive. The currentdensities were amp. per sq. ft. for D. C. and A. C. at the cathode and 5amp. per sq. ft. for D. C. and A. C. at the anode. By the same procedureas in Example IV, a 0.01'7-inch thick electroplate was formed on acopper-base material after 72 hours plating time. The coating was adense, smooth, bright deposit, having good ductility.

Example VI The same procedure and apparatus was used as in Example IV,substituting 2.5 cc. di-o-tolylurea as the organic additive. Currentdensities used were 10 amp. per sq. ft. D. C. and 12 amp. per sq. ft. A.C. at the cathode and 5 amp. per sq. ft. D. C. and 6 amp. per sq. ft. A.C. at the anode. After approximately 40 hours plating time, a 0.010-inchthick electroplate was formed on a copper-base material serving as thecathode. This coating was dense, smooth and had good ductility.

Example VII The same bath and apparatus were used as in Example IV,except that 5 cc. of dichloroethylene were used as the organic additive.Using current densities of 11 amp. per sq. ft. D. C. and A. C. at thecathode, and 5 amp. per sq. it. D. C. and A. C. at the anode, anelectroplate 0.0l9-inch in thickness was formed on a copper-basematerial serving as the cathode. The coating was found to have excellentductility.

As can be seen from the preceding examples, by utilizing the process andbath of this invention, it is possible to form aluminum coatings inthicknesses greater than 0.002 inch without treeing or formingnonadherent powders or sponge. The improved coatings have superiorproperties of hardness and ductility. Hollow articles of complex shapescan be formed by applying this coating to a low-melting metal base, andsubsequently melting out the base. The bath is long-lived and can beused over long periods of time without appreciable change in the qualityof the coatings produced thereby.

What is claimed is:

1. The method of electrodepositing a thick, dense aluminum coating on anelectrically conductive surface which comprises the steps of forming afusion of about 2 mols of aluminum chloride and about 1 mol of ethylpyridinium bromide, making a bath by dissolving said fusion in toluene,said toluene being present in an amount suificient to provide an excesslayer in volume equal to about 23 to about 45 per cent of the totalvolume, adding an organic material selected from the group consisting ofmethyl tertbutyl ether, ethyl ether, and diphenyl oxide, in an amountsufficient to achieve said thickness and density, bubbling an inert gasthrough said bath, whereby the excess solvent is continuously dispersedthroughout said bath, and simultaneously passing an electric currentthrough the bath between an aluminum anode and the electricallyconducting surface serving as a cathode.

2. An electrolyte for electrodepositing thick, dense aluminum on anelectrically conducting surface, said electrolyte comprising toluene inwhich is dissolved an organic additive selected from the groupconsisting of methyl te-rt-butyl ether, ethyl ether, and diphenyl oxide,in an amount sufficient to achieve said thickness and density, and afusion of about 2 mols of aluminum chloride and about 1 mol of ethylpyridinium bromide, said toluene being present in an amount sufiicientto provide an excess layer in volume over that required to dissolve saidfusion equal to about 20 to about 45 per cent of the total volume of theelectrolyte.

3. The method of electroforming a thick, dense aluminum article whichcomprises the steps of forming an electrically conductive base in adesired configuration, immersing said article in a bath, said bathcomprising a solution of a liquid aromatic hydrocarbon solvent in whichis dissolved an organic additive selected from the group consisting ofmethyl tert-butyl ether, ethyl ether, and diphenyl oxide, in an amountsufficient to achieve said thickness and density, and a fusion of analuminum halide and a quaternary salt of nitrogen, said solvent beingpresent in sufficient quantity to form a layer of excess solvent,continuously dispersing said excess solvent throughout the bath, passingan electric current between an aluminum anode and said conductive baseas a cathode until an aluminum coat ing is deposited thereon, removingsaid coated base from the bath, and separating said conductive base fromsaid aluminum coating.

4. The method of electroforming a thick, dense aluminum article whichcomprises the steps of forming a solid nonmetallic base in a desiredconfiguration, making the surface of said solid base conductive,immersing said surface in a bath, said bath comprising a solution of aliquid aromatic hydrocarbon solvent in which is dissolved an organicadditive selected from the group consistin of methyl tert-butyl ether,ethyl ether, and diphenyl oxide, in an amount sufiicient to achievesa-id thickness and density, and a fusion of an aluminum halide and aquaternary salt of nitrogen, said solvent being present in sufficientquantity to form a layer of excess solvent, continuously dispersing saidexcess solvent throughout the bath, passing an electric current betweenan aluminum anode and said conductive base as a cathode until analuminum coating is deposited thereon, removing said coated base fromthe bath, and separating said conductive base from said aluminumcoating.

*5. The method of electrodepositing a thick, dense aluminum coating ofan electrically conductive surface, which comprises the steps of forminga fusion of an aluminum halide and a quaternary salt of nitrogen, makinga bath by dissolving said fusion in a liquid aromatic hydrocarbonsolvent, said solvent'being present in sufiicient quantity to form alayer of excess solvent separate from said solution, adding an organicmaterial selected from the group consisting of methyl tert-butyl ether,ethyl ether, and diphenyl oxide, in an amount sufficient to achieve saidthick- 8 ness and density, maintaining the excess sol' vent dispersedthroughout the bath, and passing an electric current through thesolution between an aluminum anode and an electrically conductivesurface serving as a cathode.

6. The method according to claim 5 wherein the nitrogen salt is aN-alkyl pyridinium halide.

7. An electrolyte for electrodepositing thick, dense aluminum on anelectrically conductive surface,- said electrolyte comprising a liquidaromatic hydrocarbon solvent in which is dissolved an organic additiveselected from the group consisting of methyl tert-butyl ether, ethylether, and diphenyl oxide, in an amount sufficient to achieve saidthickness and density, and a fusion of an aluminum halide and aquaternary salt of-nitrogen, said solvent being present in sufficientquantity to form a layer of excess solvent separate from said solution.

8. The bath according to claim '7, in which the nitrogen salt is aN-alkyl pyridinium halide.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,854,634 Brode et al Apr. 19, 1932 2,446,349 Wier et a1 Aug.3, 1943 2,446,350 Wier Aug. 3, 1943 OTHER REFERENCES Safranek et al.:Plating, January 1948, pp.

3. THE METHOD OF ELECTROFORMING A THICK, DENSE ALUMINUM ARTICLE WHICHCOMPRISES THE STEPS OF FORMING AN ELECTRICALLY CONDUCTIVE BASE IN ADESIRED CONFIGURATION, IMMERSING SAID ARTICLE IN A BATH, SAID BATHCOMPRISING A SOLUTION OF A LIQUID AROMATIC HYDROCARBON SOLVENT IN WHICHIS DISSOLVED AN ORGANIC ADDITIVE SELECTED FROM THE GROUP CONSISTING OFMETHYL TERT-BUTYL ETHER, ETHYL ETHER, AND DIPHENYL OXIDE, IN AN AMOUNTSUFFICIENT TO ACHIEVE SAID THICKNESS AND DENSITY, AND A FUSION OF ANALUMINUM HALIDE AND A QUATERNARY SALT OF NITROGEN, SAID SOLVENT BEINGPRESENT IN SUFFICIENT QUANTITY TO FORM A LAYER OF EXCESS SOLVENT,CONTINUOUSLY DISPERSING SAID EXCESS SOLVENT THROUGHOUT THE BATH, PASSINGAN ELECTRIC CURRENT BETWEEN AN ALUMINUM ANODE AND SAID CONDUCTIVE BASEAS A CATHODE UNTIL AN ALUMINUM COATING IS DEPOSITED THEREON, REMOVINGSAID COATED BASE FROM THE BATH, AND SEPARATING SAID CONDUCTIVE BASE FROMSAID ALUMINUM COATING.